Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and a few cars, dirt bikes, and ATVs. In some vehicles, drum brakes are applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated by the rear is less.
Due to the fact that a drum brake's friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes, but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake self-applying nature, large friction surface contact area, and long life wear characteristics.
Drum brakes convert kinetic energy into heat by friction. While much of this heat should dissipate into the surrounding air, it can easily transfer to the brake system components. Brake drums must be large to cope with the massive forces involved, and must be able to absorb and dissipate a lot of heat. Excessive heating can occur due to heavy or repeated braking, which can cause the drum to distort, leading to vibration under braking. Another consequence of overheating is brake fade. When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, so the shoes must move farther and the driver must press the brake pedal farther.
Conventional drum designs have been known to exhibit non-uniform thermal expansion, which increases brake pulsation and unevenly distributed braking force. Brake judder and vibration are frequently experiences. In certain cases, the high service requirements of large truck drum brakes produce increased and unacceptable levels of drum cracking.
Another issue with regard to conventional drum brake designs stems from the fact that they are heavier than disc brakes, adding significant weight to a vehicle and a corresponding reduction in fuel economy.
In summary, prior art brake drums have suffered from problems associated with wear, heat and material fatigue due to stresses induced during the braking process. It would be desirable to reduce the stresses experienced by brake drums and, to increase performance and durability, while decreasing weight.
There is a need, therefore, to provide a brake drum and wheel hat assembly that reduces or eliminates the afore-mentioned problems to increase the life and reliability of the brake device, while providing an assembly that is lower in weight.